In general, structural displacements and openings caused by cracks or deformations in civil structures are on the order of a few micrometers to a few centimeters. Conventionally, mechanical systems using springs and magnetic transduction methods or other similar techniques are employed to measure large displacements such as these. These systems are not easily deployable in some structures because of the lack of conformity with structural shapes. Further, these systems tend to have a low degree of resolution with respect to the displacement measurements. Strain-gauge-type systems, such as fiber optic Bragg gratings, are more easily deployed but are highly sensitive and enjoy high resolutions, such that their strain measurement range is small—on the order of 5,000 microstrains, which translates to a fraction of a millimeter once converted to displacement over their gauge length. The displacement measurement range for these strain monitoring systems is low when they are configured to be attached to straight mechanical elements. In this type of arrangement, the displacements due to crack openings are directly transferred to the strain gauge and provide a limited displacement range, i.e., 5,000 microstrains times the length of the mechanical element providing at best a fraction of millimeter in displacement range. Higher displacement readings may damage the strain sensor, or just simply are not transduced.
It would therefore be desirable to have a fiber optic system for measuring structural displacements and crack openings in civil structures that has a greater range of measurement than conventional displacement-measuring systems employing optical fibers.
The invention provides such a displacement-measuring system. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.